It is an object of the present invention to provide an optical amplifying unit to be used for optical telecommunications. The invention also relates to an optical transmission system, more particularly a wavelength division multiplexing (WDM) optical transmission system, which uses the above-mentioned optical amplifying unit. The optical amplifying unit of the invention is also adapted to be used in analog CATV systems.
In WDM optical transmission systems, transmission signals including several optical channels are sent over a same line (that includes at least an optical amplifier) by means of wavelength division multiplexing. The transmitted channels may be either digital or analog and are distinguishable because each of them is associated with a specific wavelength.
Present-day long-distance high-capacity optical transmission systems use optical fiber amplifiers that, differently from previously used electronic regenerators, do not need OE/EO conversion. An optical fiber amplifier includes an optical fiber of preset length, having the core doped with one or more rare earths so as to amplify optical signals by stimulated emission when excited by pump radiation.
Optical fibers doped with erbium (Er) have been developed for use as both optical amplifiers and lasers. These devices are of considerable importance since their operating wavelength coincides with the third window for optical fiber communications, around 1550 nm. EP patent application No. 98110594.3 in the name of the Applicant proposes a thirty-two channels WDM optical transmission system that uses erbium-doped fiber amplifiers (EDFAs) in the wavelength bands 1529-1535 nm and 1541-1561 nm.
Several methods have been proposed to improve the system performances, for example in terms of amplification gain and amplification bandwidth.
One technique for improving the system performances consists in co-doping an erbium-doped amplification fiber with ytterbium (Yb). Co-doping an active fiber with erbium and ytterbium not only broadens the pump absorption band from 800 nm to 1100 nm, offering greater flexibility in selection of the pump wavelength, but also greatly increases the ground state absorption rate due to the higher absorption cross section and dopant solubility of ytterbium. The ytterbium ions absorb much of the pump light and the subsequent cross relaxation between adjacent ions of erbium and ytterbium allows the absorbed energy to be transferred to the erbium system. As described in Grubb et al., xe2x80x9c+24.6 dBm output power Er/Yb co-doped optical amplifier pumped by diode-pumped Nd:YLF laserxe2x80x9d, Electronics Letters, 1992, 28, (13) pp. 1275-1276, and in Maker, Ferguson, xe2x80x9c1.56 xcexcm Yb-sensitized Er fibre laser pumped by diode-pumped Nd:YAG and Nd:YLF lasersxe2x80x9d, Electronics Letters, 1988, 24, (18), pp. 1160-1161, the co-doping technique may be applied to efficiently excite fiber amplifiers and lasers through direct pumping in the long wavelength tail of ytterbium absorption spectrum. This pumping is preferably performed by means of diode-pumped solid state lasers, for example 1047 nm Nd:YLF lasers or 1064 nm Nd:YAG lasers.
Using an erbium and ytterbium co-doped amplification fiber to amplify communication signals is further described in European patent application EP 0 803 944 A2 and in U.S. Pat. No. 5,225,925. EP 0 803 944 A2 refers to a multistage Er-doped fiber amplifier (EDFA) operating in the wavelength band 1544-1562 nm and comprising a first stage that includes Er and Al and a second stage that includes Er and a further rare earth element, for example Yb. Such multistage EDFA can have advantageous characteristics in the cited wavelength band over the all-erbium amplification systems, e.g. a relatively wide flat gain region, and relatively high output power, without significant degradation of the noise figure. However, the Applicant noted that the amplifier proposed in EP 0 803 944 A2 offers no advantages in terms of number of transmitted channels, the amplification bandwidth being still limited to the relatively narrow (and largely exploited) 1544-1562 nm band. Furthermore, the Er/Yb second stage is pumped by means of a diode-pumped Nd-doped fiber laser emitting at 1064 nm. This pump source, largely used for the excitation of mono-modal amplification fiber, is relatively expensive and cumbersome.
U.S. Pat. No. 5,225,925 relates to an optical fiber for amplifying or sourcing a light signal in a single transverse mode. The fiber comprises a host glass doped with erbium (Er) and a sensitizer such as ytterbium (Yb) or iron (Fe). Preferably the host glass is a doped silica glass (e.g. phosphate or borate doped). The Applicant noted that U.S. Pat. No. 5,225,925 proposes an amplification fiber that, due to the shape of its gain curve, is particularly adapted for the transmission of a single channel at 1535 nm but is not suitable for WDM transmissions. Moreover, such an amplification fiber is adapted to be pumped by means of a diode-pumped Nd-doped fiber laser that has the above mentioned disadvantages.
Neither EP 0 803 944 A2 nor U.S. Pat. No. 5,225,925 address amplification by an Er/Yb co-doped optical amplifier of a signal in a wavelength band different from the transmission band around 1550 nm.
An improvement of Er/Yb amplification fibers has been obtained by means of the cladding pumping technique, which consist in pumping the active fiber in an inner cladding region surrounding the core, instead that directly in the core. Cladding pumping is a technique that allows high power broadstripe diodes and diode bars to be employed as efficient, low cost and small dimension pump sources for double-clad rare earth doped single-mode fibers. Output powers ranging from several hundred milliwatts to several tens of watts may be attained by this technique. A double-clad Er/Yb fiber pumped by diode arrays at 980 nm is described, for example, in Minelly et al., xe2x80x9cDiode-array pumping of Er3+/Yb3+ co-doped fibre lasers and amplifiersxe2x80x9d, IEEE Photonics Technology Letters, 1993, 5, (3), pp. 301-303. The erbium-ytterbium co-doped scheme enables much higher ground state absorption for erbium in the band about 980 nm than singly-doped erbium fibers, resulting in much shorter optimum length. The technique of inserting the pump radiation into a portion of the fiber external the core (which can be identically identified as an inner cladding or an outer core) is also described, for example, in PCT patent application WO 95/10868 and in U.S. Pat. No. 5,696,782.
Several methods have also been proposed to increase the number of channels to be transmitted. One way to increase channel numbers is to narrow the channel spacing. However, narrowing channels spacing worsens nonlinear effects such as cross-phase modulation or four wave mixing, and makes accurate wavelength control of the optical transmitters necessary. Applicant has observed that a channel spacing lower than 50 GHz is difficult to achieve in practice do to the above reasons.
Another way to increase the channel number is to widen the usable wavelength bandwidth in the low loss region of the fiber. One key technology is optical amplification in the wavelength region over the conventional 1550 nm transmission band. In particular, the high wavelength band around 1590 nm, and precisely between 1565 nm and 1620 nm, is a very promising band for long-distance optical transmissions, in that a very high number of channels can be allocated in that band. If the optical amplifier for the 1565-1620 nm band must deal with a high number of channels, the spectral gain characteristics of such amplifier are fundamental to optimize the system""s performances and costs. The use of the 1590 nm transmission wavelength region of erbium-doped fiber amplifiers in parallel to the 1530 and 1550 wavelength regions, is attractive and has been recently considered. As an additional advantage, by employing the 1590 nm wavelength region it is possible to use dispersion-shifted fiber (DSF) for WDM transmissions without any degradation caused by four-wave mixing.
Several articles in the patent and non-patent literature address amplification in the high wavelength transmission band (from 1565 nm up to 1620 nm). All these documents consider only erbium-doped fiber amplifiers.
The following documents propose several methods to enlarge the usable bandwidth to the high wavelength transmission band.
U.S. Pat. No. 5,500,764 relates to a SiO2xe2x80x94Al2O3xe2x80x94GeO2 single-mode optical fiber (having a length between 150 m and 200 m) doped with erbium, pumped by 1.55 xcexcm and 1.47 xcexcm optical sources and adapted to amplify optical signals between 1.57 xcexcm and 1.61 xcexcm.
Ono et al., xe2x80x9cGain-Flattened Er3+-Doped Fiber Amplifier for a WDM Signal in the 1.57-1.60 xcexcm Wavelength Regionxe2x80x9d, IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 9, No. 5, May 1997, pp. 596-599, disclose a gain-flattened Er3+-doped silica-based fiber amplifier for the 1.58 xcexcm band WDM signal; different fiber lengths were tested and the authors found that 200 m was the optimum length of EDF (Erbium-Doped Fiber) for constructing an EDFA with high gain and low noise.
Masuda et al., xe2x80x9cWideband, gain-flattened, erbium-doped fibre amplifiers with 3 dB bandwidths of  greater than 50 nmxe2x80x9d, ELECTRONICS LETTERS, Jun. 5, 1997, Vol. 33, No. 12, pp. 1070-1072, propose a scheme with two-stage erbium-doped fibres and an intermediate equalizer, obtaining a 52 nm band (1556-1608 nm) for a silicate erbium-doped fiber amplifier and a 50 nm band (1554-1604 nm) for a fluoride erbium-doped fiber amplifier; in the case of a silicate erbium-doped fiber amplifier, the two stages include a 50 m EDF and a 26 m EDF, respectively.
Jolley et al., xe2x80x9cDemonstration of low PMD and negligible multipath interference in an ultra flat broad band EDFA using a highly doped erbium fiberxe2x80x9d, xe2x80x9cOptical Amplifiers and their Applicationsxe2x80x9d Conference, Vail, Colo., Jul. 27-29 1998, TuD2-1/124-127 proposes a broad band EDFA which amplifies signals in the 1585 nm band using 45 m of erbium fiber and reaching a maximum external power of more than +18.3 dBm at 1570.
The Applicant has observed that a line EDFA adapted to amplify optical signals in the high wavelength band can amplify an optical signal having an input power of approximately xe2x88x9210 dBm to a maximum power value lower than 19 dBm, i.e. with a maximum gain of approximately 29 dB. An input power of approximately xe2x88x9210 dBm is a proper reference value, being typical for optical amplifiers in long-distance transmission systems. Lower input power are not recommended in that, although EDFAs have higher gains for low power input signals than for high power input signals, the ASE (amplified spontaneous emission) in this case increases to values such that the signal to noise ratio becomes to low. On the contrary, signal input powers over xe2x88x9210 dBm, obtainable for example to the detriment of transmission fiber length, tends to saturate the gain, leading to an undesirable waste of energy. An optical transmission system using EDFAs and transmitting sixty-four channels between 1575 nm and 1602 nm would provide a maximum power per channel, at the output of the line EDFAs, of about 0.2 dBm and would limit in practice the maximum span length to less than 100 km.
The Applicant has further observed that in an erbium-doped active fiber of a predetermined length, the curve of the gain vs. erbium concentration has an increase up to a maximum, corresponding to an optimum value of erbium concentration, and then a decrease. Higher gains are obtainable only increasing the length of the active region doped with erbium, i.e. increasing the active fiber length. Long-haul WDM optical transmission systems for the high wavelength band using conventional erbium-doped active fibers require fiber lengths of a few hundred meters to reach a relative high gain. Actually, special erbium-doped active fibers having a larger core diameter are used, which allow obtaining a relative high gain with fiber lengths down to 30-40 m.
The Applicant has found that, in the 1565-1620 nm band, transmission systems including erbium-ytterbium co-doped amplifiers provide very high performances, in particular they provide higher performances with respect to erbium-only doped optical amplifiers. In particular, the Applicant has found that an optical amplifying unit including an erbium-ytterbium co-doped fiber amplifier (single-stage or multi-stage), optimized in terms of length and doping, can provide very high gain and a very flat amplification band (xc2x10.5 dB) in a wavelength region having a width of at least 27 nm and situated above 1565 nm. More in detail, the Applicant has found that an optical amplifying unit including an optimized erbium-ytterbium co-doped fiber amplifier can provide, in the 1575-1602 nm wavelength region, an output signal power up to 23 dBm in response to an input of approximately xe2x88x9210 dBm power. The Applicant has further found that such a high power gain can be efficiently reached by including a pre-amplifier, preferably an erbium-doped pre-amplifier, into the optical amplifying unit.
Furthermore, the Applicant has observed that, for an erbium-ytterbium co-doped fiber, the region of increase in the gain vs. erbium concentration curve is much more extended than for erbium-doped fibers, and has found that, in the 1565-1620 nm band, relatively short active fibers may be used. The Applicant has found that, depending on system parameters such as the signal power at the input of the amplifying unit and the erbium concentration in the active fiber of the amplifying unit, an optimum fiber length of the active fiber can be chosen to minimize the gain tilt in the considered wavelength band.
Moreover, the Applicant has found that, in the considered high-wavelength band, high pump performances can be obtained using erbium-ytterbium double-clad fiber, taking advantage of the multi-mode pumping mechanism.
The Applicant has further found that the above-described amplifying unit can be advantageously used in a long-haul WDM transmission system to obtain high performances in transmissions in the wavelength region up to 1620 nm. In particular, the Applicant has found that a wide-band long-haul WDM transmission system can be realized by subdividing the wavelength transmission band in three sub-bands corresponding to 1529-1535 nm, 1541-1561 nm and 1575-1602 nm, and amplifying the 1575-1602 sub-band by means of optical amplifying units including at least an Er/Yb co-doped amplifier, preferably combined with an Er-doped pre-amplifier. Such a wide band allows, for example, the efficient transmission of sixty-four channels spaced by 50 GHz.
Moreover, the Applicant has found that, thanks to the high gain achievable by means of Er/Yb co-doped amplifiers in the high-wavelength band, a WDM transmission system in the 1575-1602 nm band may include fiber spans having length greater than or equal to 130 km between subsequent amplification stages.
According to a first aspect, the present invention relates to an optical transmission system including an optical transmitting unit adapted to transmit an optical signal in a transmission wavelength band above 1570 nm, an optical receiving unit to receive said optical signal, an optical fiber link optically coupling said transmitting unit to said receiving unit and an optical amplifying unit coupled along said link, said optical amplifying unit having an amplification wavelength band including said transmission wavelength band and comprising an input for the input of said optical signal from said link, an output for the output of said optical signal into said link and an optical amplifier interposed between said input and said output to amplify said optical signal, said optical amplifier including an amplification fiber, a pump source for generating pump radiation and an optical coupler for optically coupling said pump source and said amplification fiber, characterized in that said amplification fiber includes an optical fiber co-doped with erbium and ytterbium.
In particular, said optical amplifying unit has a power gain greater than 29 dB when said optical signal has an input power of at least xe2x88x9210.5 dBm and wavelength within said amplification wavelength band. Preferably, said optical amplifying unit has a power gain of at least 31 dB when said optical signal has an input power of at least xe2x88x9210.5 dBm and wavelength within said amplification wavelength band. More preferably, said optical amplifying unit has a power gain of at least 33 dB when said optical signal has an input power of at least xe2x88x9210.5 dBm and wavelength within said amplification wavelength band.
Preferably, the width of said amplification wavelength band is at least 15 nm and more preferably at least 27 nm.
Preferably, said lower wavelength limit of said amplification wavelength band is greater than or equal to 1575 nm.
Preferably, said optical amplifying unit has a gain variation lower than 1 dB within said amplification wavelength band.
Preferably, said optical transmission system further includes an optical pre-amplifier interposed between said input of said optical amplifying unit and said optical amplifier to pre-amplify said optical signal.
Preferably, said amplification fiber has a core having a concentration of erbium between approximately 600 ppm and 1000 ppm.
Preferably, said amplification fiber has a core having a ratio between ytterbium and erbium concentrations between approximately 5:1 and 30:1.
Preferably, said amplification fiber has a length lower than 30 m and more preferably lower than 13 m.
Preferably, said amplification fiber is a double-clad fiber having a core, an inner cladding surrounding the core and an outer cladding surrounding said inner cladding.
Preferably said optical link includes optical fiber spans having a length of at least 130 km.
In a second aspect, the invention relates to a method for transmitting optical signals, comprising:
generating an optical signal having a wavelength in a wavelength band, said wavelength band having a lower limit greater than 1570 nm;
feeding said signal to an optical link;
amplifying said signal along said optical link;
receiving said optical signal from said optical link;
characterized in that said step of amplifying comprises feeding said signal into an active fiber co-doped with erbium and ytterbium.
Preferably, said step of feeding comprises feeding said signal into one end of said active fiber, said active fiber having a length, an erbium concentration and an ytterbium concentration such that the power gain of said optical signal at the opposite end of said active fiber is at least 25 dB.
More preferably, said step of feeding comprises feeding said signal into one end of said active fiber, said active fiber having a length, an erbium concentration and an ytterbium concentration such that the power gain of said optical signal at the opposite end said active fiber is at least 31 dB.
Preferably, said wavelength band has a width of at least 27 nm.
In a further aspect, the invention relates to an optical amplifying unit for amplifying optical signals in an optical transmission system, said optical amplifying unit having an amplification wavelength band with a lower wavelength limit greater than 1570 nm and with a width of at least 15 nm, and including an amplification fiber, a pump source for generating pump radiation and an optical coupler for optically coupling said pump source to said amplification fiber, characterized in that said amplification fiber includes an optical fiber co-doped with erbium and ytterbium.
The foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of this invention.